1. Technical Field
The present invention relates to a power conversion device which can supply a stable voltage to a load even though the voltage of an alternating current power source fluctuates.
2. Background Art
FIG. 18 is a diagram for illustrating a form of a power conversion device disclosed in PTL 1 (identified further on). The power conversion device converts the voltage of an alternating current power source to a direct current voltage once, converts the converted direct current voltage to a predetermined alternating current voltage again, and supplies the predetermined alternating current voltage to a load. In FIG. 18, 1 is a single-phase alternating current power source, 2 a capacitor, 3 a direct current power source circuit, 4 an inverter circuit, 5 a filter circuit, and 6 a load. The alternating current power source 1 includes a power source terminal R and a power source terminal S as terminals which output the alternating current voltage. Hereafter, the power source terminal R will be referred to as the terminal R, and the power source terminal S will be referred to as the terminal S. The capacitor 2 is connected between the terminal R and terminal S of the alternating current power source 1.
The direct current power source circuit 3 is formed of capacitors Cp and Cn, switching elements Qp and Qn, and a reactor Lr. The capacitors Cp and Cn are connected in series. The connection point of the capacitors Cp and Cn, as well as being connected to the terminal S of the alternating current power source 1, is connected to a neutral point terminal O of the direct current power source circuit 3. The switching elements Qp and Qn are connected in series. The series circuit of the switching elements Qp and Qn is connected in parallel to the series circuit of the capacitors Cp and Cn. The reactor Lr is connected between the terminal R of the alternating current power source 1 and the connection point of the switching elements Qp and Qn.
By the switching elements Qp and Qn being turned on and off, the capacitors Cp and Cn are charged to an equal voltage using the voltage of the alternating current power source 1. The voltage of the charged capacitors Cp and Cn is a voltage higher than the amplitude of the alternating current voltage output from the alternating current power source 1. Further, the direct current power source circuit 3 outputs the positive voltage of the capacitor Cp from a positive side terminal P, and outputs the negative voltage of the capacitor Cn from a negative side terminal N, with the neutral point terminal O as a reference.
The inverter circuit 4 is configured of the series circuit of switching elements Q1 and Q2 connected in series and bidirectional switches BS1 and BS2. Diodes are connected in reverse parallel to the respective switching elements Q1 and Q2. The connection point of the switching elements Q1 and Q2 is connected to one output terminal U of the inverter circuit 4. The other output terminal V of the inverter circuit 4 is connected to the neutral point terminal O. The series circuit of the switching elements Q1 and Q2 is connected between the positive side terminal P and negative side terminal N of the direct current power source circuit 3. The bidirectional switch BS1 is connected between the terminal R of the alternating current power source 1 and the output terminal U. The bidirectional switch BS2 is connected between the neutral point terminal O and the output terminal U.
The inverter circuit 4 selectively turns on and off the switching elements Q1 and Q2 and the bidirectional switches BS1 and BS2. By this operation, a predetermined alternating current voltage is output between the output terminal U and the output terminal V. Hereafter, “between the output terminal U and the output terminal V” will be referred to as “between the output terminals U and V”. The alternating current voltage output between the output terminals U and V is a pulse-width modulated pulse train voltage.
The filter circuit 5 is configured by connecting a reactor Lf1 and a capacitor Cf1 in series. The filter circuit 5 is connected between the output terminals U and V of the inverter circuit 4. The load 6 is connected across the capacitor Cf1.
The filter circuit 5 removes a high frequency component from the alternating current pulse train voltage output by the inverter circuit 4. Consequently, a sinusoidal alternating current voltage is supplied to the load 6.
PTL 1 discloses a technology wherein the power conversion device outputs a predetermined voltage lower than the voltage of the alternating current power source 1. The power conversion device, by alternately turning on and off the bidirectional switches BS1 and BS2, outputs a predetermined voltage, which is lower than the voltage of the alternating current power source 1, between the output terminals U and V. Also, the power conversion device can output a predetermined voltage higher than the voltage of the alternating current power source 1. The power conversion device, by selectively turning on and off the switching elements Q1 and Q2 and the bidirectional switch BS1, outputs a predetermined voltage, higher than the voltage of the alternating current power source 1, between the output terminals U and V.